Chassis dynamometers are used in a wide variety of applications, particularly in connection with the testing of motor vehicle engine emissions pursuant to Environmental Protection Agency (EPA) emissions regulations for motor vehicles. Such dynamometers typically incorporate one or more "rolls" which are driven by one or more wheels of a test vehicle. The rolls are typically coupled to an input shaft of some form of power absorption, or exchange device which provides a controlled degree of rolling resistance to the rolls to simulate load and inertia forces normally encountered during vehicle operation. A vehicle engine must overcome inertia forces in order to accelerate or decelerate the vehicle. The engine must also overcome breakaway frictional and rolling frictional forces, as well as adapt to wind forces, generally referred to as road load forces. Dynamometers are designed to simulate the conditions that a vehicle is faced with during actual road operation.
In an effort to ensure that the dynamometer accurately measures torque, exhibits little hysteresis, and accounts for drift, calibration of the dynamometer is required on a periodic basis.
A dynamometer's torque measuring system may be calibrated using the deadweight method. The deadweight method applies known torques on the dynamometer calibration arm, against which the dynamometer output is calibrated. The known torques are produced by calibration weights. Accordingly, a series of calibration weights are applied to a calibration arm or arbor and weight hanger to simulate positive and negative torques. Unfortunately, since the dynamometer is located below the floor level of the test cell in a confined space, the application of the weights onto the calibration arm is an awkward and potentially hazardous process.
With previous dynamometer designs, calibration of the dynamometer required a person to lower, for instance, 50 pound weights, into a confined space, and then subsequently place the weights one at a time on the weight hanger for incremental calibration. The calibration operator would incrementally place weight on the hanger to simulate the varying loads applied during vehicle testing. As an example, for dynamometers testing light-duty vehicles, weights in the range of 650 pounds would have to be applied to the calibration hanger. For complete calibration, an incremental weight, for example 50 pounds, is applied to the calibration hanger, after which the calibration weight is steadied on the calibration arm to obtain a stable reading. For each incremental weight up to the recommended total weight, for example 650 pounds, a calibration reading is taken.
Accordingly, for each of these calibration weights, under the previous designs, a calibration operator would have to carry the calibration weight, for example 50 pounds, and place it on the calibration hanger, until the desired load, for example 650 pounds, was attained. These calibration designs thus posed a significant safety hazard to the employees in charge of calibration, as this process required lifting, sliding and manipulating a substantial amount of weights, with the constant danger of back or other related injuries resulting from the heavy load and strain. In view of this awkward, time-consuming and straining procedure, calibrations of dynamometers were only done when absolutely necessary.